The present invention relates to a method for joining a member made of a metallic oxide such as sapphire to another member.
A crystal of a metallic oxide such as zirconia (ZnO2) or xcex1-phase aluminum oxide such as sapphire has good mechanical properties, heat resistance, and corrosion resistance and is used as a member of various types of devices. For example, a pressure sensor having a housing and the like made of sapphire is proposed. This pressure sensor is a capacitive pressure sensor which detects a pressure by detecting an electrostatic capacitance and has a housing comprised of a base with a predetermined space and a diaphragm arranged above the space of the base, a stationary electrode arranged in the base, and a movable electrode fixed to the diaphragm. The base and diaphragm are made of sapphire.
Sapphire, i.e., a corundum (xcex1-phase aluminum oxide) has high thermal stability and can hardly dissolve in acid and alkaline solutions, so that it is widely used as a refractory material, insulator, abrasive, and the like. By using a member with the properties described above for the housing, even when an object to be measured is a corrosive fluid, the above-described pressure sensor can measure the pressure of the fluid by directly receiving it at the diaphragm.
The above-described pressure sensor chip is used as a pressure sensor by fixing to a pedestal, as shown in FIG. 7. As shown in FIG. 7, a pressure sensor chip 701 is placed in a recess 703 formed at a central portion of a pedestal 702 made of, e.g., glass to face a diaphragm portion upward in FIG. 7. Terminals 705 of electrode pins 704 extending through the bottom surface of the pedestal 702 are formed on the bottom surface of the recess 703 of the pedestal 702, which are respectively connected to the wires of electrodes led to the lower surface of the base of the chip 701. A vent port 706 communicating the bottom surface of the pedestal 702 is formed in the bottom surface of the recess 703.
A cover plate 707 having an opening portion at the central portion presses the periphery of the upper surface of the chip 701, thereby fixing it to the recess 703. The cover plate 707 is joined to the upper surface of the pedestal 702 by using glass melted once and fixed to the pedestal 702. The cover plate 707 and chip 701 are airtightly sealed with each other such that no fluid to be measured brought into contact with the upper surface of the chip 701 enters the recess 703 around the chip 701.
To ensure pressure measurement accuracy, the cover plate 707 is made of sapphire which is the same material as that of the chip 701 to obtain the same degree of deformation, due to temperatures, as that of the chip 701, such that no stress acts on the chip 701. To airtightly seal the chip as described above by excluding dissimilar metal joint as much as possible, the abutting surfaces of the cover plate 707 and chip 701 are directly joined. To join the two members made of sapphire as described above, their joint surfaces are mirror-polished and abutted, and the two sapphire materials to be joined are heated while applying a pressure therebetween, thereby joining them strongly without using an adhesive or the like.
In the above-described direct joining, however, since the abutting surfaces of the two sapphire members must be mirror-polished to a roughness of 0.3 nm or less, the expensive member (cover plate) is required. When a general adhesive is used, no joint surfaces must be mirror-polished. However, since a different material is interposed between the two members, a stress occurs, and the same corrosion resistance and thermal stability as in sapphire are not obtained at the joint portion. This restricts the application range.
As described above, when two or more members made of a metallic oxide are to be conventionally joined to form a component, the members are directly joined to take full advantage of the material properties of the members, but direct joining increases a cost. In contrast, when the two members are joined by using an adhesive or the like, the component is formed at a low cost, but the full advantage of the material properties of the member is not taken.
The present invention has been made to solve the above problem, and has as its object to allow to join members made of a metallic oxide such as sapphire at a low cost without degrading the properties of the members.
In a method for joining according to an aspect of the present invention, a solution layer formed from a solution in which a compound containing a metal and oxygen of a metallic oxide is dissolved is formed on a joint surface of a base made of the metallic oxide, the solution layer is heated to form, on the joint surface of the base, an adhesion layer containing the metal and oxygen in an intermediate phase state which is unstabler than the metallic oxide and in which an energy level tends to change into a lower level, another member is arranged on the adhesion layer, and a pressure is applied between the base and another member while heating the adhesion layer to cause phase transition of the adhesion layer in the intermediate phase state to the metallic oxide with a stabler phase than that of the adhesion layer, thereby joining the base to another member.
According to this method for joining, the base and another member are joined without any material other than the material of the base therebetween.
In the method for joining described above, when the adhesion layer is heated while applying a pressure between the base and another member, an electric field may be simultaneously applied to the adhesion layer to promote, e.g., oxygen ion diffusion to promote phase transition.
In the method for joining described above, the metallic oxide is xcex1-phase aluminum oxide, and the adhesion layer is made of aluminum oxide with a phase other than an xcex1 phase.
In the method for joining described above, the compound containing the metal and oxygen is a salt containing the metal and an acid, an organometallic compound, or a hydroxide of the metal. In addition, the compound containing the metal and oxygen is boehmite. Further, the aluminum oxide with a phase other than the xcex1 phase is one material selected from the group consisting of xcex3-, xcex8-, -, xcexa-, xcex5-, "khgr"-, xcex4-, and "sgr"-phase aluminum oxides or a compound thereof.